1. Field of the Invention
The present invention relates generally to a motionless wide steering-range optical transmitter device comprising core planar wave guide based liquid crystal beam steering device(s) in series of state-of-the-art fine steering device(s) for simultaneously steering multiple beams of light into arbitrary and separate directions with a field of regard close to 4π.
2. Description of the Prior Art
Optical beam steering devices, also termed as optical transmitters thereinafter, are critical devices for applications in wireless optical communication, image transferring, and remote measurement. A beam steering or transmitter device is such a device that alters or scans the propagation direction or position of a beam of light via a certain controlling means. Current state-of-the-art beam steering technologies include at least the following categories, i.e., electro-mechanical, electro-optical, electro-acoustic, and thermo-optical. A typical electromechanical beam steering device (BSD) is Gimbal based beam steering device that steers beam of light via mechanically rotating the gimball (Robert A. Gill and Robert J. Feldmann, “Development of Laser Data Link for Airborne Operations”, http://www.dodccrp.orz/Proceedings/DOCS/wcd00000/wcd00099.htm).One single gimball BSD steers one beam of light over a wide solid angle in the sequence manner at a relatively slow speed.
Microelectromechanical system (MEMs) based beam steering device is another type of electromechanical device (Hung Nguyen, John Guo-Dung Su, Hiroshi Toshiyoshi, and Ming C. Wu, “Device Transplant of Optical MEMS For Out Of Plane Beam Steering”, MEMs 2001; B. Mukherjee, “Optical Communication Networks”, U C Davis, ECS Lecture 259 2b, 2002). MEMs technology employs micro-mirror or micro-mirror array to deflect beam of light for steering and has advantages of relatively fast speed, compact size, and reasonable steering range.
Electro-acoustic beam steering device is a stationary BSD which steers beam of light via light diffraction from the medium grating generated within the acoustic medium by the acoustic wave from the transducer (Benjamin L. Brown, “MULTI-BEAM FAST-STEERED TWEEZERS”; http://atomsun.harvard.edu/tweezer/multi.html; www.isomet.com/FinalWebSite/PDFDocs/AO%20Sheets/1250C-2BS-943.pdf; Acoustic BSD-2; Andrea Fusiello and Vittorio Murino, “Calibration Of An Optical-Acoustic Sensor”, Machine Graphics & Vision, Vol. 9, P.207, 2000). Electro-acoustic BSD randomly steers beam of light fast and consumes significant electric power with limitations in beam programmability.
Electro-optical beam steering devices include liquid crystal (LC) optical phased array beam steering device (LCOPA BSD) (Paul McManamon, Terry A. Dorschner, david L. Corkum, Larry J. Friedman, Douglas S. Hobbs, Nichael Holz, Sergey Liberman, Huy Q. Nguyen, Daniel P. Seler, Richard C. Sharp, and Edward A. Watson, “Optical Phasded Array Technology”, Proc. IEEE, Vol. 84, 268, 1996; Phil Bos, “Liquid crystal Based Optical Phased Array For Steering Lasers”, STAB-Kickoff meeting, August, 2000), liquid crystal spatial light modulator (SLM) based beam steering device (Bruce Winker, “Liquid Crystal Agile Beam Steering”, STAB-Kickoff meeting, August, 2000), liquid crystal blazed grating based beam steering device (Xu Wang, Daniel Wilson, Richard Muller, Paul Maker, and Demetri Psaltis, “Liquid-crystal blazed-grating beam deflector”, Appl. Opt., Vol. 39, P.6545, December 2000), as the typical examples.
In LCOPA BSD, a phase profile is imposed on an optical beam as it is either transmitted through or reflected from the phase shifter array. The imposed phase profile steers the beam of light. The array of optical phase shifters is realized through lithographic patterning of an electrical addressing network on the superstrate of a liquid crystal waveplate. Refractive index of the liquid crystal changes sufficiently large to realize full-wave differential phase shifts can be effected using low voltages applied to the liquid crystal phase plate electrodes.
Liquid crystal blazed grating based beam steering device comprises a substrate having a Poly(methyl methacrylate) (PMMA) blazed grating and a thin layer of nematic liquid crystal (LC) sandwiched in between the grating substrate and plane substrate, both of which have Indium-Tim-Oxide (ITO) electrodes. The electric field applied to the ITO electrodes electrically drives the LC to change the phase information of the illuminating light, or the refractive index for extraordinary light. In the absence of the electric field, the refractive indices of the PMMA substrate and LC are different, and strong diffraction is produced by the refractive index-phase difference of this OFF state. When an electric field is applied, the refractive index of the LC is decreased to a certain point where index matching occurs between the PMMA and the LC. Light passes through the device without changing its propagation direction.
Electro-optical beam steering devices based on optical wave guide have been invented. For example, in the device invented by Lin Sun, et al (“Polymeric waveguide prism-based electro-optic beam deflector”, Opt. Eng. 40(7), 1217–1222 (July 2001), optical beam is deflected via the principle that based on the fact that the propagation direction of the light beam can be changed by inducing an index pattern in the EO medium by applying an electric field. The triangular structure of the top electrode induces a triangular variation of index in the core layer made of EO polymeric material. Light propagating through the deflector deviates from its original path at the interfaces between adjacent regions, because of the difference in the indices of refraction. A light beam propagating within the planar wave guide formed by the polymer layers will thus have its direction of propagation modified in a manner similar to that of a beam passing through a set of physical prisms.
Another example of electro-optical switching/steering device in wave guide format is the polarization sensitive optical switch/beam steering prototypes designed and realized based on liquid crystal (LC) integration in planar wave guides (http://people.na.infn.it/˜abbate/gruppo/Waveguides.htm). Different geometries was exploited and both nematic and ferroelectric liquid crystals (FLC) was used, but the operation principle was always the same: a liquid crystal cell is realized in place of the waveguide core or cladding; a weak applied voltage (few Volts) is able to reorient liquid crystal (LC) molecules, change the layer refractive index and affect light propagation. As an particular example, a rectangular basin, as deep as core film and rotated with respect to the direction of light propagation, is etched on the waveguide by ordinary photolithography. It was proposed to fill the basin with a ferroelectric liquid crystal, which offers faster response time (in the ms range) than ordinary nematic liquid crystals. Right alignment is assured by a rubbed polymeric layer deposited on lower surface of a glass cover. FLC is chosen so that, with no applied voltage, its refractive index coincides with core one and a transmission state is obtained; switching on electric field, LC refractive index becomes lower so that total reflection takes place at core-basin interface and beam deflection is obtained. This device is polarization sensitive.
As an example of thermo-optical switching/steering device in wave guide format, polarization insensitive planar-waveguide switch employing liquid crystal as the switching elements has been disclosed by John Thackara (“Planar Waveguide Switch and Optical Cross—Connect, WO 02/31558, International Publication Date: 18 Apr., 2002). Different from the mentioned electro-optical device, this device switches beam of light via thermal means rather than electric means. This switching device is also regarded as a beam steering device from which the steered beam steered has the same propagation direction. The core layer of the switch's planar waveguide contains a narrow trench filled with a liquid crystal that exhibits positive birefringence. When held at a temperature that is a few degrees above a threshold value (or “clearing point”), the liquid crystal's isotropic refractive index matches that of the core layer, allowing nearly complete optical transmission through the switch. Cooling the liquid crystal temperature to below the clearing point, however, both polarizations of the incident optical signal are totally reflected from the trench.
Most these state-of-the-art beam steering devices are effective in steering single beam of light.